The present invention concerns a control system for a machine for digging and/or drilling earth and a digging and/or drilling machine comprising such a system, able to be used in particular in the field of civil engineering.
In order to contain or reinforce earth it is known to make temporary or permanent buried walls, commonly called “diaphragms”, or walls, or diaphragm walls.
In order to make these digging and/or drilling machines are used that are equipped with digging heads, like for example the hydraulic bucket, the cabled bucket, the hydromilling machine and so on. Such digging heads are generally moved, or in any case supported during normal operation of the machine, by a load-bearing cable, which is wound or unwound around the drum of a motor winch.
In order to carry out digging that is as vertical as possible, it may be required during drilling that the digging device not be completely rested on the bottom of the excavation, but that it always be held by the lifting cable. In this way, the digging device is always hung from the lifting cable and under the action of the force of gravity behaves like a sort of “plumb line” or “pendulum”.
In the case of the hydromilling machine, the debris resulting from the digging is extracted and taken to the surface by means of a centrifugal pump. The maximum flow rate of this pump is defined at the design stage and consequently sets a maximum velocity of descent of the hydromilling machine during drilling. Descending at high velocities, the pump would not be able to evacuate all of the debris that is dug by the toothed wheels of the hydromilling machine and consequently the hydromilling machine would rest on the bottom of the excavation, on the materials accumulated and not evacuated. In this way, the mill would tilt with respect to the vertical, altering the quality of the diaphragm. Moreover, the digging teeth would sink too far into the earth, with the possibility of jamming the wheels in the ground. This can cause the digging operations to slow down, as well as causing the wheels or the motors that move them to break, making it necessary to extract the mill from the excavation for it later to be reset. Consequently, there will be a lengthening of the drilling times and of the relative costs.
However, the toothed wheels are not always able to produce an amount of debris that allows the pump to be exploited to the fullest. When dealing with earth that is very difficult to dig, the velocity of descent is no longer dictated by the maximum flow rate of the pump but by the digging velocity of the wheels.
It is obvious that by making the hydromilling machine descend at velocities that are too low, verticality is maintained, but the toothed wheels are prevented from going into the earth and advancing in the drilling, negatively affecting digging performance.
In particularly difficult earth, like for example rocky layers with high resistance to compression and not very fractured, the optimal velocity of descent can take on very small values, of the order of a few centimeters per minute.
Moreover, in this case it is necessary to use very heavy digging heads to increase the digging efficiency so that actual production depends on the thrust that is exerted on the cutting element (digging tooth) and on how close it is to the optimal value for that particular type of earth. Such a weight thus forces the use of winches that must be able to develop a very high pulling force through the load-bearing cable. By using known winches it is very complicated to move the digging heads at very low velocities. The winches currently known, indeed, are equipped with a single hydraulic motor, coupled with the drum through a gearmotor. The hydraulic motors typically used in these winches are piston motors, commonly known as high-speed motors, because they are not suitable for operating at low velocities generally below 50 revs per minute. Indeed, when the rotation velocity of the motor reaches values close to the aforementioned lower limit, the efficiency of the motor and the torque able to be delivered by it undergo a drastic drop.
In light of the above, in order to reach very low velocities of descent with the hydraulic motors described above, the load-bearing cable is usually relayed a predetermined number of times through many pulleys arranged between the winch and the digging head. In this way there is a multiplication of the pulling force, very often even more than quadrupled. However, this involves a substantial increase in the amount of cable to be stored on the drums (which is at least proportional to the depth and the number of relays on the tackle), forcing the use of very bulky winches that make it difficult to transport the digging machine and increase its production cost.
Moreover, the use of a large number of pulleys involves longer descent and ascent times of the digging heads at the start and end of digging, respectively. Therefore, what may be acceptable during digging, by increasing the number of relays to control very low velocities and neglecting the consequent increase in the amount of cable, would no longer be acceptable with reference to the ascent and descent times of the head.
One only has to think that the movement systems currently known have maximum velocity of descent and ascent of the digging and/or drilling apparatus of about 6-8 m/min and, consequently, the use of such movement systems at depths of 250 m involves descent and ascent times of about 30 min. Increasing the number of relays involves a further lengthening of these times, which would become unacceptable.
There are currently also known digging machines provided with two winches to wind or unwind the load-bearing cable of the digging head, wherein each winch is actuated by a high-speed hydraulic motor. Being able to store double the amount of lifting cable on the two drums, it is possible to arrange a greater number of relays of the cable and to make the digging head move forward at lower velocities. However, these machines with two winches continue to offer a relatively limited field of variation of the velocity, limited maximum velocities of forward motion and low ability to finely adjust the velocity of forward motion at low operating velocities.
Lifting systems are known which provide for the use of two winches with a single cable. Such a system is described in U.S. Pat. No. 6,926,259. In practice, in it an end of the cable is connected to the first drum, relayed through pulleys up to the digging device and connected with the other end to the second drum. This provision makes it possible to store the cable on two drums rather than on just one. In this way it is possible to keep the size of the drums small, having to contain at most a little over half of the cable.
A system of mobile pulleys allows the tackle to be modified by moving the pulleys from the trolley to the vertical guide tower. In this way, it is possible to “manually” and discreetly change the range of velocities of the digging device.
The use of small sized winches also prevents the maximum pulling force able to be delivered to the cable from drastically reducing. Indeed, it decreases with a law proportional to the diameter of the drum. However, this solution has the drawback of having to move the pulleys from the mast to the trolley in order to be able to move the digging device at very low velocities. This process is certainly slow and complicated and not very safe, also requiring a very complex manual intervention.
Patent EP0538630 shows a winch equipped with two hydraulic motors, at least two epicycloidal gearings and a juxtaposition gear. The solution makes it possible to increase the range of velocities covered by the winch, to add the powers of the two motors and to finely adjust the velocity of the cable.
However, this remains a single-drum solution, which does not allow enormous amounts of cable to be stored and that is wasteful due to the gearings used. Moreover, it is not interchangeable with classical winches with a single motor given the much greater lateral dimensions.
The U.S. Pat. No. 7,503,548 describes a lifting device formed from two winches that can be synchronised through the action of two belts.
Each of the two winches is moved by a hydraulic motor with reducer. Both of the drums are connected through a belt to a special synchronisation shaft. In normal conditions, the two winches behave like two independent standard winches. By placing the hydraulic channel 76 under pressure the coupling is activated by friction between the two synchronisation wheels that are constrained to rotate together on the synchronisation shaft. In this way, the winches are mechanically forced to rotate in the same direction and with the same velocity. This characteristic is useful to obtain a synchronised lifting of the same load. The belts are toothed, and as alternatives to them it is possible to use toothed wheels. The two winches, when the synchronisation device is activated, are constrained to rotate in the same direction, i.e. they both unwind or collect cable.
This solution, having two drums, makes it possible to store the cable necessary for our purpose, but it does not allow the low velocities that would be needed to be obtained.